Numerous metal chlorides can be thermally transformed into metal oxides and hydrogen chloride. In the art it is known to introduce metal chlorides into furnaces in the solid, molten or dissolved state for this purpose. In many cases, the thermal transformation or conversion of metal chlorides is a link in the chain of production of the metal from the metal ore, an ore concentrate or a metallurgical intermediate.
A special case of the thermal decomposition of a metal chloride is the working-up of depleted pickling baths obtained from the pickling of metals with hydrochloric acid.
Among the processes used for the regeneration of such pickling baths is a roasting technique whereby the pickling liquid, arising from the pickling of iron or steel, is sprayed in a hot combustion gas. This technique has, to a large measure, been replaced by a fluidized-bed process. The pickling acid is directly introduced into the fluidized bed or the metal chloride is first crystallized from the pickling acid and is then introduced into the fluidized bed, e.g. in a slurry.
An important advantage of the fluidized-bed technique is that it directly produces a granular, abrasion-resistant dust-free metal oxide with high chloride purity (freedom from chloride). As distinct from the spray-roasting process in which the thermal transformation of the metal chloride to the corresponding metal oxide must be effected with a residence time of only a few seconds, the solid particles of a fluidized bed may remain in the reaction zone for a number of hours to permit the development of metal oxide products of large particle size and to permit complete thermal transformation to the metal oxide.
The importance of obtaining a complete thermal transformation of the metal chloride to the metal oxide has led to domination of fluidized-bed processes in the field of the regeneration of pickling acids.
However, it has not been possible by earlier techniques of operating such fluidized beds to avoid certain disadvantages. For example, when sand constitutes the material of the fluidized bed, a substantial part of the reaction product is a finely divided metal oxide which is entrained out of the fluidized-bed furnace with the reaction gases. The entrained dust particles of metal oxide can be recovered and handled only with difficulty in a dry state. Removal of the fine-particle component by wetwashing and like techniques has the disadvantage that a slurry is produced which can be worked up to recover the metal oxide only with difficulty.
When the fluidized-bed process has been carried out with a metal oxide constituting the solid phase of the bed, continuous operation over a period of several days leads to an increase (growth) in the particle size and a concomitant sharp reduction in the turbulence of the fluidized bed. With the growth of the particles constituting the bed there is an increase in the open or free space in the bed and a reduction in the completeness of the reaction. As a result, a dust-like metal oxide component is produced.
It has hitherto been necessary, with both the metal-oxide and the sand fluidized beds, to pass the effluent dust-containing gas stream through a high-efficiency wet precipitator in which the hot waste gas is brought into direct contact with a metal chloride solution and the latter is thereby concentrated. The resulting effluent product has an increased metal content although some of the oxide remains undissolved. In extreme cases, the proportion of the undissolved oxide dust is so large that difficulties result and interfere with the normal operation of the process. To eliminate the latter disadvantage, it has been proposed to introduce the reaction gas from the fluidized bed into a cyclone to recover a dust which is returned to the fluidized bed. In practice, this arrangement has been found to improve the results only slightly. Control of the particle growth within the bed is generally not possible since, in large measure, the fine particles are continuously carried off before they have achieved the desired increase in size.